1. Field of the Invention
The present invention is directed to a device for compensating for flow resistance in a ventilator/respirator, and to a ventilator/respirator containing such a device.
2. Description of the Prior Art
As used herein, the term "ventilator/respirator" also refers to other equipment for providing respiratory support, such as anesthetic machines etc.
When a patient is connected to a ventilator/respirator, a series of resistances to flow, in the form of gas lines, an endotracheal tube, dehumidifier, flowmeter, bacterial filters etc., are introduced. They jointly make it harder for the patient to breathe, in particular to exhale. This is unpleasant to the patient.
A system, primarily for compensating for the resistance to flow in the endotracheal tube, is described in the article "Automatic compensation of endotracheal tube resistance in spontaneously breathing patients" by Fabry et al., Technology and Health Care, 1 (1994) 281-291, but the corresponding principle can also be applied to the entire expiratory section.
A fan is used to generate a constant negative pressure on the outlet side of the expiratory section, i.e. downstream from the expiratory section as viewed from the patient. An expiratory valve can be regulated by a valve system, exposing the expiratory channel to a larger or smaller part of this constant negative pressure. Pressure is measured in the expiratory channel at the expiratory valve and by the patient. If known, previously measured and calibrated endotracheal tubes are used, and pressure in the patient's lungs can be calculated from the flow measured near the patient. The valve system is regulated so a programed reference value for lung pressure is maintained. If the calculated value for lung pressure is too high, the valve system is caused to increase negative pressure, and vice-versa.
This known compensation system takes into account the circumstance that flow resistance is also related to flow rate. A disadvantage of this system is a risk of excessive amounts of gas being evacuated from the patient's lungs, which would therefore collapse. This is a grave threat to the patient, so compensation systems of various kinds have seldom been used in practice.
Another shortcoming is that the system does not work with "unknown" endotracheal tubes, i.e. endotracheal tubes whose flow resistance and pressure drop properties have not been specifically determined.